I also think that Australias arepretty happy following the non-Aussie sides...
For me, its not a problem that the Pacific nation teams are mostly heritage players as it just shows RL as a multi-cultural game (and its damn near impossible for nations of below 100k to field a competitive team). And, like fans not following their local NRL club, i think fans will find a reason to back teams other than the Roos (their own heritage, an holiday destination, love of the local culture, favorite players).
Only having one Aussie team is fine. Fans can easily jump on another bandwagon. The Roos are the giants that everyone wants to bring down; like ManU or Real Madrid, having one team always winning isnt necesarily bad.